Controlled temperature chamber



March 14, 1967 NAGY, ET E 3,309,498 I CONTROLLED TEMPERATURE CHAMBEROriginal Filed Dec. 51, 19s: 2 Sheets-Sheet 1 I INVENTORS. JOHN NAGY,JR. Y ROMAN I. ANDRUSHKIW their ATTORNEYS March 14, 1967 N JR ET AL,3,309,498

CONTROLLED TEMPERATURE CHAMBER Original Filed De c. 31, 1963 I 2Sheets-Sheet 2 INVENTORS. JOHN NAGY, JR. a BY ROMAN a. ANDRUSHKIW theirATTOR/VfYS United States Patent I n 3,309,498 CONTROLLED TEMPERATURECHAMBER John Nagy, Jr., Union, and Roman I. Andrushkiw, Newark, N.J.,assignors to D aystro m, Incorporated, Murray Hill, NJ., a corporationof Texas Continuation of abandoned application Ser. No. 334,745, Dec.31, 1963. This application Mar. 11, 1966, Ser. No. 533,729.

Claims. (Cl. 219-210) 3,309,498 Patented Mar. 14, 1967 amount of lightstriking the cell in accordance with the temperature of the chamber. Thechamber is thus maintained at the desired constant temperature.

The apparatus automatically adjusts for variations of supply voltage andambient temperature, but provision is made for additional automaticcompensation against such changes by adjusting the light energy radiatedby the lamp. The temperature sensitive element also includes a dialarrangement for indicating the temperature of the chamber.

All of the above is more fully explained in the detailed description ofa preferred form of the invention which follows, this description beingillustrated by the accompanying drawings wherein:

FIG. 1 is an elevational view in section of the overall apparatusaccording to the invention;

FIG. 2 is an enlarged elevational view of a portion of the apparatus ofFIG. 1;

FIG. 3 is an enlarged fragmentary view, taken along the line 33 of FIG.2 and looking in the direction of the arrows;

ence junctions or temperature sensitive solid statedevices may require aconstant temperature environment for proper operation. V V

In general this is achieved by mounting the element in a closed housingor chamber incorporating a heat source controllable by a temperatureresponsive device. One of the devices which have been employed for thispurpose in the past is a thermal switch, which uses a bimetallic contactelement as the switch arm to turn an electrical heating element on andoff as the temperature in the chamber falls below or rises above thedesired temperature, respec: tively. Prior art chambers controlled inthis manner necessarily oscillate or hunt about the desired temperature,since the heat source is turned on or olT only after an incrementalchange in temperature within the chamber. The use of a mechanicalcontact switch also introduces unreliability due to corrosion and arcingat the contacts. Furthermore, variations in line voltage can affect thetemperature regulation, necessitating the use of a regulated powersupply in some instances. 7 This, in turn, increases the complexity andpower drain of the chamber, while reducing its reliability. 7 7

Accordingly, his the primary object of the present invention to providean improved controlled temperature chamber which avoids thedisadvantages of known constructions. p 7

Another object of the invention is to provide a novel method andapparatus for automatically controlling the heat energy supplied to sucha chamber.

A further object of the invention is to provide an im-, proved methodand apparatus for controlling the tempera ture of such a chamber whichautomatically compensates for variations in ambienttemperature andsupply voltage. .Still another object, of the invention is to provide animproved method and apparatus for controlling the temperature of achamber with a high degree of stability and reliability.

These and other objects and advantages .of the invention are attained byutilizing a photosensitive device for controlling the heat source andadjusting the amount of radiant energy striking the photosensitivedevice in accordance with the temperature of the oven.

In one form of the apparatus according to the invention, a lampilluminates a photoconductive cell which controls the gain or anamplifier circuit. The circuit includes a power transistor whose powerdissipation whileoperating supplies the heat for thechamber. Atemperature sensitive element continuously senses the temperature withinthe chamber and has a movable member disposed between the lamp and thephotoconductive cell to' control the 21, fastened such FIG. 4 is anenlarged fragmentary view, taken along the line 4-4 of FIG. 2 andlooking in the direction of the arrows;

FIG. 5 is an enlarged sectional view of the thermometer assembly used inthe embodiment shown in FIG. 1;

FIG. 6 is a top plan view of the embodiment shown in FIG. 1; and I FIG.7 is a schematic illustration of an electrical circuit of the preferredform of the invention.

Referring now to FIG. 1 of the drawing, the chamber 10 whose temperatureis to be controlled, is surrounded by a housing comprising a baseportion 12 having an upstanding peripheral flange 13, and an inner can14 adapted to make a friction fit with the peripheral flange of theinner cover. Both the inner can and the base portion are made preferablyof a good thermal conductor such as aluminum. Secured to the baseportion12 is a heat sink 15 formed of a block of a good thermalconductor such as aluminum. The heat sink is recessed to form a chamber16 adapted to receive with a close tolerance fit a power transistor 17,which is the heating element for the chamber 10. The element 11 shown indotted line, represents the transistor or other device requiring thecontrolled temperature environment. The housing is mounted on thebottomplate 18 by a screw 19 which passes through the base portion 12 andengages the heat sink 15, thereby securing it to the inner cover. A topplate 20 is mounted in spaced relation to the bottom plate 18 by aplurality of long spacers as by screws to the top and bottom plates.

As seen more clearly in the enlarged detail of FIG. 2, which is a viewof the apparatus from a direction opposite to FIG. 1, a lamp assembly 22is mounted in an aperture 23 in the top plate 20. The lamp assemblyincludes a small incandescent bulb 24 whose body has a flange portion 25which abuts one side of the top plate. A spring clip 26, which makes atight frictional fit with the lamp body, has a plurality of protrudingprongs 27 which bear against the other side of the top plate to hold thelamp assembly firmly in position in the top plate. The base of the lampis received in a socket 28, having two terminals 29 to which aresoldered the leads toconnect the lamp to the rest of the electricalcomponents.

Theupper face of the bulb 24 is coated with an opaque material 30, suchas black lacquer, with the exception of a narrow straight slit 31through which light is emitted, as may be seen in FIG. 3. Line-s 32 arescribed on the top plate for accurately aligning the slit in the desireddirection, for a purpose which will be explained hereinafter. Whenaligned, the flange 25 of the lamp is cemerited to the top plate 20 asat *33 to prevent relative angular movement between the lamp and the topplate.

Also mounted on the top plate 20 is a photo conductive cell assembly 34.This assembly includes a bracket 35 secured to the top plate by aplurality of screws 36 and nuts 37. Depending from the bracket 35 is astop 35a, the function of which will be explained below. Aphotoconductive cell 38 is cemented to the bracket 35 so as to be inspaced relation to and coaxial with the face of the bulb 24. Only anarrow linear strip 39 of the photoconductive cell (see FIG. 4) issensitive to light, and the cell is positioned on the bracket 35 withthe aid of a scribed line 40 and a notch 41 on the bracket, so that thestrip 39 is directly opposite and parallel to the slit 31 of the bulb24. A pair of leads 42 connect the photoconductive cell to the otherelectrical components, passing through suitable apertures in the bracketand top plate.

Referring now to FIGS. 1 and 5, adjustably mounted on the top plate 20is a thermometer assembly 46 including a case 47 to which is securedcoaxially thereof a cylindr-ical tube 48. Fixed to the tube at itsjunction with the case is a threaded sleeve 49. A bore 50 in the topplate receives the sleeve, and the thermometer assembly is releasablysecured to the top plate by a lock Washer 51 and two nuts 52 whichthreadedly engage the sleeve 49.

A scale plate 53 is adjustably secured to the inside of the case 47, andan appropriate temperature scale is registered on the scale plate asseen in FIG. 6. A pointer 56 having a relatively narrow indicating tip58 and a broad tail portion 57 is mounted above the scale plate on avertical shaft 55 which is rotatably supported by a bushing 43 withinthe tube 48. Also disposed within the tube 48 is a coiledbimetallictemperature sensitive element 54, one end of which is coupled to thelower end of shaft 55 and the other end of which is fastened to a hollowbushing 44 fixed within the tube 48. The element 54 angularly displacesthe shaft 55, and consequently the pointer 56, in accordance withvariation in temperature as is well known in the art. The indicating tip58 of the pointer indicates the temperature to which the bimetallicelement is exposed on the scale 53.

A glass cover 59 is secured to the case 47 by a suitable clamping ring45, and the half of this cover over the nonindicating portion of thescale plate is painted with an opaque material such as black lacquer,indicated at 60. Aligned openings 53a and 47a are provided in the scaleplate 53 and case 47, respectively, below the coated portion of thecover 59; see FIG. 6. As shown in FIG. 1, the thermometer assembly 46 ismounted on the top plate 20 so that the bracket 35 and the portion ofthe bulb 24 protruding from the top plate extend through the openings53a and 47a. These openings are of such size and shape as to permit arotation of the thermometer assembly 46 on the top plate 20 ofapproximately 30 Without striking either the bulb 24 or the bracket 35.The broad tail portion 57 of the pointer 56 is suitably bent to permitit to pass freely between the bulb 24 and the photoconductive cell 38without striking either.

With the thermometer assembly 46 mounted on the top plate 20, the tube48 extends through aligned bores in the inner can 14, the heat sink 15,the base portion 12 and the bottom plate 18. A spacer 69 of thermallyconductive material and a spacer 68 of thermally nonconductive materialeach slidably fit around the tube 48, abutting each other, and the innercan 14 and the sleeve 49, respectively. Thus when the thermometerassembly is mountedon the top plate, the inner can is retained in afully seated engagement with the base portion. The spacer 68 is of suchlength as to extend to the upper end of the temperature sensitiveelement 54.

An outer cover plate 70 having an upstanding peripheral flange 71 issecured in spaced relation to the bottom plate by a plurality of shortspacers 72. An outer cylindrical can 74, having a pluraltiy of laterallyextending mounting ears 75 at its lower end also is provided with acircular opening at its upper end for accommodating the indicator dialportion of the apparatus. The lower end of the can 74 is secured byscrews to the flange on the cover plate 70 to maintain the elements inproper assembly. Thermally insulating material 79 is provided within theouter can and cover plate as indicated in,

FIG. 1.

The electrical connections for the power transistor 17 and the device 11are brought out by insulated terminals 62 passing through the baseportion 12 and bottom plate 18, as shown in FIG. 1. A suitable socket 63for the device 11 is also mounted thereon. If desired,non-heatdissipating elements of the electrical circuitry may be mountedon the underside of bottom plate 18.

The outer cover plate is also provided with a plurality of terminals 64to which are connected the various circuit components and conductors tobe discussed in connection with FIG. 7. The leads from the lamp assembly22 as well as the elements within the chamber 10 are also coupled viathe terminals 64 to the power source and the remainder of the circuit.These circuit elements may be mounted above the lower end of the can 74to permit mounting or may extend below as shown for mounting in asuitable aperture.

The electrical components for effecting the automatic temperaturecontrol are illustrated schematically in FIG. 7. Power is connected fromA.C. source 82 to the primary coil of a step-down transformer 83designed to provide a potential difierence of, for example, 56 voltsA.C. across the output terminals 84 of the secondary coil. To theseoutside terminals are connected the positive terminals of two crystaldiodes 80, which may be the type designated CER 69, for example. Thenegative terminals of the two diodes are connected together at aterminal 85, and a terminal 86 is connected to a center tap on thetransformer secondary coil. As will be recognized, the transformer 82and the diodes comprise a full wave rectifier, the output of whichappears across the terminals and 86, with terminal 85 being at positivepolarity.

A resistor 87 is connected between the terminal 86 and a terminal 88,the bulb 24 being connected between the terminals 88 and 85. Theresistor 87 may have a value of approximately 400 ohms, its purposebeing to reduce the voltage applied across the bulb to approximately 14volts. The bulb is designed to operate at 28 volts and would normallyhave a life of about 3000 hours. By operating it at 14 volts, its lifebecomes in excess of 100,000 hours. When operated at this reducedvoltage, the bulb emits a reddish light. The operation of the apparatusis nevertheless still satisfactory, since the photoconductive cell 38 issensitive to red light.

A thermistor 89 is also connected between the terminals 85 and 88 and ispositioned in thermal contact with the resistor 87, both the thermistorand the resistor being disposed away from the chamber, preferably on thecover plate 70. The thermistor is chosen to have a negative temperaturecoefficient, and its resistance varies in inverse proportion to thepower dissipated in the resistor 87 and to the ambient temperatureexternal to the oven housing.

A resistor 90 is connected between the terminal 86 and a terminal 91,and the photoconductive cell 38 connects the terminals 85 and 91. Theresistance of the photoconductive cell varies with the amount of lightincident thereon, as is well known to the art, and consequently, thepotential of the terminal 91 with respect to the terminal 86 varies inaccordance with the amount of light striking the photoconductive cell.The resistor 90 may have a value of 33,000 ohms, and the photoconductivecell 38 may be of cadmium sulfide and have a resistance of 1 megohm ormore in the absence of light, its resistance decreasing as the amount ofincident light increases.

A resistor 92 is connected between the terminal 86 and the emitter of apower transistor 94, whose collector is joined to the terminal 85. Thebase of the transistor is connected to the terminal 91. The resistor 92serves as a limiting resistor if one is required for the transistor andother circuit components chosen. With a power transistor of the typedesignated 2N2339 and the values of the other components as statedabove, the resistor 92 is unnecessary. For purposes of illustration, thethermometer assembly 46 is indicated symbolically at (1) on FIG. 7,thermally coupling the power transistor 94 (the heater for the chamberwith the pointer 56, whose broad tail portion 57 is pivoted so as topass between the lamp 24 and the photoconductive cell 38.

In operation, during warmup the chamber 10 is below the desiredtemperature, and the pointer will initially be at one extremity of itsangular displacement. The broad tail portion of the thermometer pointerthus is displaced from its central position in which it blocks lightfrom the lamp from reaching the photoconductive cell and the latter isfully exposed to the light emitted through the slit 31 of the lamp. Theresistance of the cell is thereby decreased, raising the potential ofthe terminal 91. Under these conditions, the base of the transistor isforward biased with respect to its emitter to an extent rendering thetransistor fully conductive. A maximum of power is thus dissipated bythe transistor, tending to raise the temperature in the chamber 10.

As the chamber nears the desired temperature, the broadtail portion 57of the pointer begins to shade partially the photoconductive cell 38,changing the bias on the transistor to reduce the current through it andits power dissipation. Finally, when the desired temperature is reached,the broad tail portion so shades the cell that the power dissipationprovided by the transistor is sufiicient to supply only the heatnecessary to maintain the temperature inside the chamber constant at thedesired value. If for any reason the temperature in the chamber shouldrise above the desired value, the broad tail portion of the thermometerpointer would move further to prevent any light emitted through the lampslit 31 from reaching the photoconductive cell. The stop 35a (best seenin FIGS. 1 and 2) prevents the pointer from being further displaced soas to permit more light to reach the photoconductive cell as thetemperature increases.

The opaque coating 60 on the glass cover minimizes the amount ofincident light at the cell, and with the cell completely shielded by thebroad tail portion ofthe pointer from the bulb, the cells resistancewould be at its maximum value, in which case the transistor 94 becomesreversed biased and effectively non-conductive. The transistor willremain non-conductive until the temperature begins to fall below thedesired temperature.

The amplification provided by the power transistor 94 results in a verybrief warmup period for the apparatus and provides excellent temperatureregulation in the chamber 10, since a slight change in the amount oflight striking the photocell results in a large change in powerdissipation. The aligned light emitting and light sensitive strips onthe bulk and photocell respectively, and their proximity to each other,also contribute to the sensitivity of the temperature control. However,oscillation or hunting of the circuit about the desired temperature isavoided by reducing to a minimum the thermal resistance path between theheater (the power transistor 94) and the temperature sensitive element54 of the thermometer. The heat sink 15, the inner can 14 and the baseportion 12 are all preferably made oftaluminurn, an excellent heatconductor. In addition, the areas of contact between the inner can 14and base portion 12, the heat sink and the thermometer tube 48, theinner can and the thermometer tube, and the power transistor and theheat sink are all coated with silicone grease to insure a low thermalresistance path between the transistor and the thermometer tube.

Variations in line or supply voltage and ambient tem perature areautomatically taken into account by the apparatus of the invention sinceboth input voltage and temperature are used to control the circuitoperation. An increase in line voltage will increase the lamp brightness and thus the current through the transistor. The resulting increasein temperature in turn causes the broad tail portion of the pointer toshade the photocell to a greater extent to maintain the powerdissipation substanially constant. The converse effect is provided upona decrease in line voltage.

Changes in the chamber temperature resulting from ambient temperatureconditions or the heat dissipation of the device 11 in the chamberdirectly affects the thermom eter element 46 and are thus taken intoaccount in the normal operation of the control circuit.

A refinement in chamber temperature regulation against variations inambient temperature and line voltage is provided by the thermistor 89.The thermistor is located eX- ternally of the chamber 10 and a rise inambient 'tem perature will decrease the resistance of the thermistorbecause of its negative temperature coefi'icient. Since the thermistoris connected in parallel with the bulb 24, its decrease in resistancelowers the current through the lamp. This results .in less light beingemitted by the lamp, a higher resistance in photoconductive cell 38, andless power dissipated by the transistor 94. Similarly, a drop in ambienttemperature will cause a corresponding increase in heat dissipated bythe transistor. A rise in line voltage will cause an increase in voltageacross the resistor 87, increasing the power dissipated thereby.Inasmuch as the thermistor 89 is in thermal contact with the resistor87, the temperature of the thermistor will rise and its resistance willfall, decreasing the current through the lamp and, therefore, thecurrent through the transistor 94. Similarly, a drop in line voltagewill cause a corresponding increase in the current through thetransistor.

As has been previously described, the thermometer assembly 46 isadjustably secured to the top plate 20, and by loosening the two nuts 52the thermometer assembly 46' may be rotated with respect to the bulb 24and the photoconductive cell 38. Such rotation will change the value oftemperature at which the chamber 10 will be maintained. Accordingly, theapparatus of the invention may be used to determine accurately thecharacteristics of temperature sensitive circuit components, such as thezero temperature coefficient of diodes. The pointer tip 58 alwaysindicates the temperature within the oven chamber, permitting suchdeterminations as well as providing a check on the operation of thechamber during warmup, the effects of any gross changes in line voltage,and the like. It is to be understood that the apparatus of the inventionmay be easily modified to provide a multitude of controlled temperatureranges. Thus a thermoelectric cooler may be employed to maintain thechamber at room temperatureor below.

While the fundamental novel features of the invention have been shownand described, it will be understood that various substitutions, changesand modifications in the form and details of the apparatus illustratedand its manner of operation may be made by those skilled in the artwithout departing from the spirit of the invention. For example, aphotovoltaic cell responsive to the light emitted by the bulb could beemployed to control the power dissipated by a resistor disposed withinthe chamber. Moreover, the. photoconductive cell, connected to a sourceof electric potential, could be disposed in the heat sink, and the bulbcould be placed above the tail of the thermometer pointer, the bulb,thermometer pointer tail and cell being aligned as before, and the innercan being provided with a transparent top. Furthermore, aliquid-in-glass thermometer could be disposed between the bulb and thephotosensitive cell, the liquid being opaque and the thermometer bulbextending into the chamber. All such variationsand modifications,therefore, are included within the intended scope of the invention asdefined by the following claims.

We claim:

1. Apparatus for controlling the temperature of an enclosed chambercomprising, means forming an enclosed chamber, a heat source including apower dissipating transistor disposed within said chamber, the amount ofheat dissipated by said transistor being variable over a continuousrange between predetermined minimum and maximum values and dependentupon the magnitude of the current flow therethrough, a coiled,bimetallic temperature sensing element in said chamber, a light sourcehaving means forming a straight slit for providing illumination along aline of limited length, a photocell having a light sensitive stripcoextensive with said line of illumination, said light source andphotocell mounted exteriorly of said chamber with said slit and saidlight sensitive strip spaced apart and facing each other in alignedrelationship, opaque vane means of relatively broad extent with respectto said line of illumination mechanically coupled to said temperaturesensing element, said vane means adapted to be angularly moved by saidelement and interposed between said light source and photocell to shadethe latter to an extent dependent upon the temperature within saidchamber, thereby to make the amount of light reaching said photocelldependent upon the chamber temperature, and electrical circuit meansincluding said photocell for varying the current through saidtransistor.

2. Apparatus for controlling the temperature of an enclosed chambercomprising, means forming an enclosed chamber, a heat source including apower dissipating transistor disposed within said chamber, the amount ofheat dissipated by said transistor being variable over a continuousrange between predetermined minimum and maximum values and dependentupon the magnitude of the current flow therethrough, a coiled,bimetallic temperature sensing element in said chamber, a light sourceproviding illumination along a line of limited length, a photocellhaving a light sensitive surface coextensive with said line ofillumination, said light source and photocell mounted exteriorly of saidchamber with said line of illumination and said light sensitive surfacespaced apart and facing each other in aligned relationship, opaque vanemeans of relatively broad extent with respect to said line ofillumination mechanically coupled to said temperature sensing element,said vane means adapted to be angularly moved by said element andinterposed between said light source and photocell to shade the latterto an extent dependent upon the temperature within said chamber, therebyto make the amount of light reaching said photocell dependent upon thechamber temperature, and electrical circuit means including saidphotocell for varying the current through said transistor, saidtransistor being connected in an amplifier circuit and said photocellbeing coupled to its bias circuit.

3. Apparatus according to claim 2 further including in said amplifierbias circuit a thermally variable impedance element responsive toambient temperature, where- 8 by the heat provided in said chamber isadjusted in accordance with the ambient temperature.

4. Apparatus according to claim 1 wherein said vane means includes apointer extension formed thereon and there is further provided a visibleindicia plate disposed adjacent said pointer extension, whereby avisible indication correlated with the chamber temperature is pro-'vided.

5. Apparatus for controlling the temperature of an enclosed chambercomprising, means forming an enclosed chamber, a heat source including apower dissipating transistor disposed within said chamber, the amount ofheat dissipated by said transistor being variable over a continuousrange between predetermined minimum and maximum values and dependentupon the magnitude of the current fiow therethrough, temperature sensingmeans in said chamber, a light source for providing illumination along aline of limited length, a photocell having a light sensitive surfacecoextensive with said line of illumination, said light source andphotocell mounted exteriorly of said chamber with said line ofillumination and said light sen-sitive surface spaced apart and facingeach other in aligned relationship, opaque vane means of relativelybroad extent with respect to said line of illumination controlled bysaid temperature sensing means and adapted to be interposed between saidlight source and photocell to shade the latter to an extent dependentupon the temperature Within said chamber, thereby to make the amount oflight reaching said photocell dependent upon the chamber temperature,electrical circuit means including said photocell for varying thecurrent through said transistor over said range in accordance with theamount of light reaching said photocell, said transistor being connectedin an amplifier circuit and said photocell being coupled to its biascircuit, a thermally variable impedance element responsive to ambienttemperature in said amplifier bias circuit, whereby the heat provided insaid chamber is adjusted in accordance with the ambient temperature, anda voltage supply for said light source and said amplifier circuit, saidamplifier circuit including an impedance coupled to said voltage sourceand dissipating heat in accordance with the magnitude of the supplyvoltage, said thermally variable impedance element being mounted inthermal contact with said impedance, whereby the heat supplied to saidchamber is stabilized with respect to changes in supply voltage.

References Cited by the Examiner UNITED STATES PATENTS 2,218,502 10/1940Breitenstein 23669 2,870,318 1/1959 Ford 219363 2,954,479 9/1960Cibelius 219501 3,071,676 1/1963 Van Sandywk 2l9501 3,079,484 2/1963Shockley et a1. 219-501 RICHARD M. WOOD, Primary Examiner.

C. L. ALBRITTON, Assistant Examiner.

1. APPARATUS FOR CONTROLLING THE TEMPERATURE OF AN ENCLOSED CHAMBERCOMPRISING, MEANS FORMING AN ENCLOSED CHAMBER, A HEAT SOURCE INCLUDING APOWER DISSIPATING TRANSISTOR DISPOSED WITHIN SAID CHAMBER, THE AMOUNT OFHEAT DISSIPATED BY SAID TRANSISTOR BEING VARIABLE OVER A CONTINUOUSRANGE BETWEEN PREDETERMINED MINIMUM AND MAXIMUM VALUES AND DEPENDENTUPON THE MAGNITUDE OF THE CURRENT FLOW THERETHROUGH, A COILED,BIMETALLIC TEMPERATURE SENSING ELEMENT IN SAID CHAMBER, A LIGHT SOURCEHAVING MEANS FORMING A STRAIGHT SLIT FOR PROVIDING ILLUMINATION ALONG ALINE OF LIMITED LENGTH, A PHOTOCELL HAVING A LIGHT SENSITIVE STRIPCOEXTENSIVE WITH SAID LINE OF ILLUMINATION, SAID LIGHT SOURCE ANDPHOTOCELL MOUNTED EXTERIORLY OF SAID CHAMBER WITH SAID SLIT AND SAIDLIGHT SENSITIVE STRIP SPACED APART AND FACING EACH OTHER IN ALIGNEDRELATIONSHIP, OPAQUE VANE MEANS OF RELATIVELY BROAD EXTENT WITH RESPECTTO SAID LINE OF ILLUMINATION MECHANICALLY COUPLED TO SAID TEMPERATURESENSING ELEMENT, SAID VANE MEANS ADAPTED TO BE ANGULARLY MOVED BY SAIDELEMENT AND INTERPOSED BETWEEN SAID LIGHT SOURCE AND PHOTOCELL TO SHADETHE LATTER TO AN EXTENT DEPENDENT UPON THE TEMPERATURE WITHIN SAIDCHAMBER, THEREBY TO MAKE THE AMOUNT OF LIGHT REACHING SAID PHOTOCELLDEPENDENT UPON THE CHAMBER TEMPERATURE, AND ELECTRICAL CIRCUIT MEANSINCLUDING SAID PHOTOCELL FOR VARYING THE CURRENT THROUGH SAIDTRANSISTOR.